The present invention relates to water-absorbing polymer structures, a process for treatment of the surface of water-absorbing polymer structures, the surface-treated, water-absorbing polymer structures obtainable by this process, a composite comprising a water-absorbing polymer structure and a substrate, a process for production of a composite, the composite obtainable by this process, chemical products such as foams, formed bodies, and fibers comprising water-absorbing polymer structures or a composite, the use of water-absorbing polymer structures or of a composite in chemical products as well as the use of a combination of an oxide of a metal and of a metal salt for treatment of the surface of superabsorbing polymer structures.
Superabsorbers are water-insoluble, crosslinked polymers, which are capable of absorbing, and retaining under a given pressure, large quantities of aqueous liquids, in particular body fluids, such as urine or blood, by swelling and forming hydrogels. Because of these characteristic properties, these polymers principally find applications by incorporation into sanitary articles, such as, for example, baby diapers, incontinence products, or sanitary napkins.
The production of the superabsorbers generally occurs by radical polymerization of acid groups-carrying monomers in the presence of crosslinkers. In this way, by the choice of the monomer composition, the crosslinkers and the polymerization conditions and the processing conditions for the hydrogel obtained after the polymerization, polymers with different absorption properties can be prepared. Further possibilities are offered by the production of graft polymers, for example by using chemically modified starches, celluloses, and polyvinyl alcohol according to DE-OS 26 12 846.
DE 40 20 780 C1 discloses the post-treatment of superabsorbing polymers by post-crosslinking of the surfaces of the polymer particles. By means of the post-crosslinking of the surface of the water-absorbing polymer particles, in particular the absorption capacity of the polymer particles under the action of pressures is increased.
DE 199 09 653 A1 and DE 199 09 838 A1 describe powdery, surface post-crosslinked polymers which absorb water-aqueous or serous liquids or blood, which are based on acid groups-carrying monomers and which has been coated with a surface post-crosslinking agent and a cation in aqueous solution and post-crosslinked. The polymers disclosed in this state of the art have advantageous absorption properties compared to previous polymers, in particular a high permeability.
The surface post-crosslinking of the polymer structures disclosed in the above-cited state of the art leads to an increase of the absorption capacity under a pressure; however, a reduction of the retention capacity is generally linked with this surface post-crosslinking. Conversely, an increase of the retention capacity effected, for example, by a low degree of primary crosslinking, leads to reduced absorption capacity of the polymer structure under pressure. A reduced retention capacity of the polymer structure has, however, the disadvantage that hygiene articles, such as, for example, diapers, which comprise these polymer structures, are characterized by a decreased absorption and retention capacity of body fluids, while a reduced absorption under pressure in particular leads to the hygiene articles not being able to absorb liquids sufficiently while under a pressure, such as a pressure caused, for example, by a sitting diaper wearer.
Polymer structures which unite a good retention behavior and a good absorption capacity under a pressure could possibly be obtained by subjecting an originally only lightly crosslinked polymer particle, which, because of the low degree of crosslinking, has a comparatively high retention, to a surface post-crosslinking. This has, however, the disadvantage, that the superabsorbers obtained in this way are characterized by a high content in soluble parts, since the content in soluble parts increases with a decreasing degree of crosslinking. An increasing content in soluble parts is, however, linked with a decreased skin tolerance, so that hygiene articles comprising polymer structures with a high content of soluble parts are of concern from a dermatological viewpoint. In order to maintain the content in the soluble parts in the polymer structures as low as possible, a minimum amount of primary crosslinking in the polymer structure is desirable.
The present invention had the object of overcoming the disadvantages arising from the state of the art.
In particular, the present invention has the object of providing superabsorbers that are characterized by, in addition to excellent retention properties, also by good absorption properties under a pressure, as well as advantageously also by a good skin tolerance.
Furthermore, the present invention has the object of providing a process with which advantageous superabsorbers of this type, as well as a composite comprising these superabsorbers, can be prepared. In particular, it should be possible by this process to improve the retention and the absorption capacity of the superabsorber under a pressure independently from each other.
The present invention also had the object of providing hygiene articles with a high content in water-absorbing polymer structures, which are characterized by a particularly good skin tolerance and furthermore good absorption properties, in particular a good absorption capacity for body fluids.